This invention relates to transponders and in particular transponders for transmitting coded data to an interrogating transmitter/receiver and to monitoring apparatus including such a transponder. Such a transponder finds use in vehicle identification, in remote sensing of information relating to animals and can be used for security purposes.
Transponders are known from a variety of applications which in response to an interrogation signal internally generate a signal for retransmission. Such transponders suffer in certain applications from the disadvantage that they require a relatively large power source for generating the reply signal. It is also known in connection with vehicle identification to place reflective strips which are read by scanning to enable the vehicle in question to be identified. Such systems generally require a relatively large area devoted to the coded reflective strips, which in certain applications are impracticable.
It has also been proposed to use the information carrying signal to power the transponder. This has the disadvantage that the power levels which must be transmitted to the transponder in order to operate it are high. This, in turn causes difficulties, especially when one is working with carrier signals at high frequencies since the transmitted power levels may be sufficient to cause injury to human beings or animals who pass through the beam of the carrier signal. This is a problem which many countries are tackling by limiting, or proposing to limit, the powers which can be transmitted at particular frequencies.
It is an object of the present invention to provide a transponder which will not suffer from the above disadvantages.